Events can be missed or double-counted by fixed time step systems

[alice-i-cecile]

Bevy version

Bevy 0.9 (and will still exist for Bevy 0.10)

What you did

1. Send an event in an ordinary system.
2. Read the event in a fixed timestep system.

What went wrong

Events are sometimes missed, and sometimes double counted.

Additional information

This occurs because events are only double-buffered, and a clean up system is run each frame via Events::::update_system.

If there are many passes of CoreSchedule::Update for each pass of CoreSchedule::FixedUpdate (due to short frame times), events will be missed. If the opposite is true, events will be double-counted.

#6183 is closely related, but not entirely the same, as the same problem occurs with the Input and Axis resources.

[alice-i-cecile]

The cheatbook has a page on this problem, and some suggested workarounds: https://bevy-cheatbook.github.io/patterns/manual-event-clear.html

[jabuwu]

I've tried to make a small game using FixedUpdate in (unreleased) Bevy 0.10, and I briefly mentioned pitfalls in Discord, but I'll recap here.

The following problems need to be ironed out. I don't think any of them necessarily need to be "fixed" and they can all be worked around rather easily, which speaks to the strengths of Bevy, I think. But barring fixing them, we really need a page in the book or elsewhere dedicated to these problems, ways to work around them, and best practices with FixedUpdate in general. The cheatbook entry on this issue, as mentioned above, is very helpful.

#7691 (this issue)
#6183 (inputs can be missed in FixedUpdate)
#7835 (add a CoreSet to FixedUpdate)
#7836 (update GlobalTransform in FixedUpdate)

My vote is they all get addressed in a way that makes for the easiest developer experience, which is sometimes antithetical to performance. For example, updating GlobalTransform multiple times in a frame (and potentially many times due to the nature of FixedUpdate). So, I acknowledge my vote is probably controversial.

[starwolfy]

It would be great if FixedUpdate would also enjoy the missed event warning feature.

[paholg]

It would be great if there were at least a workaround for built-in events. For example, I am trying to create a fully deterministic game with bevy and bevy_rapier.

However, bevy_rapier uses RemovedComponents for cleanup, and puts the necessary system in the main schedule, even when using FixedUpdate: https://github.com/dimforge/bevy_rapier/blob/0ea000b2938e6b55659023f1b14844cb51c10b69/src/plugin/plugin.rs#L218-L219

As far as I can tell, to make this deterministic, I would have to set a custom runner that calls World::clear_trackers in FixedUpdate instead of at the end of App::update. Given how much code is in winit_runner, I would like to avoid this.

[alice-i-cecile]

This is now resolved: Events now integrate with the fixed update.